The present invention relates to a power supply apparatus which can be incorporated in arm-worn type or other type of portable electronic equipment and which is provided with a power generating device of such a type as the one that is capable of converting the kinetic energy of a human body into alternating current by making use of the motion of a rotary weight.
Portable electronic equipment has been devised and put to practical use which makes it possible to obviate the need for replacing a battery or to eliminate the battery itself by incorporating a power generating device in small electronic equipment suited for carrying such as a wristwatch device. FIG. 11 shows the schematic configuration of a wristwatch device 10 incorporating a power generating unit 1 as an example. The portable electronic equipment (wristwatch device) 10 is provided with a rotary weight 13 which swings in the case of the wristwatch device, a wheel device 11 which transmits the rotary motion of the rotary weight 13 to an electromagnetic power generator, and a rotor 14 and a stator 15 making up the electromagnetic power generator 12; when the rotor 14 which has been magnetized to have two poles rotates, an electromotive force is generated in an output coil 16 of the stator 15, and an AC output can be taken out. Further, a power supply apparatus 20 of the portable electronic equipment is equipped with a rectifier circuit 24 which rectifies the alternating current output from the power generating unit 1 and supplies it to a large-capacity capacitor 5 and a processing unit 9; connected to the output side thereof are the large-capacity capacitor 5, which serves as a charging unit, and the processing unit 9. Hence, the processing unit 9 connected to the power supply apparatus 20 is able to operate a mounted timing function 7, etc. by the power of the power generating unit 1 or the power that the large-capacity capacitor 5 has discharged. Therefore, the portable electronic equipment is able to continuously operate the processing unit 9 even in the absence of a battery, thus implementing electronic equipment that permits the use of the processing unit at any place whenever required and that is also capable of solving the problems involved in discarding batteries.
In the electronic equipment shown in FIG. 11, since the power supplied from the built-in power generating unit 1 is of alternating current, it is rectified through the rectifier circuit 24 of the power supply apparatus 20 before it is charged in the large-capacity capacitor 5, and it turns into the operating power of the processing unit 9 equipped with an IC, etc. The power supply apparatus 20 shown in FIG. 11 employs the rectifier circuit 24 which temporarily charges the power, which has been half-wave rectified using two diodes 25 and 26, in an auxiliary capacitor 27 to perform boosting and rectification. For these diodes 25 and 26, silicon diodes are employed; as shown in FIG. 12, they have a forward voltage Vf of about 0.5 V to about 0.6 V with respect to a forward current If. Hence, power W1 obtained by rectifying power W0 supplied from the power generating unit 1 through the rectifier circuit 24 will be as shown below because there is a loss attributable to the forward voltage Vf of the diodes making up the rectifier circuit 24:
W1=xcex7cxc3x97W0xe2x80x83xe2x80x83(1)
xcex7c=V1/(V1+2xc3x97Vf)xe2x80x83xe2x80x83(2)
where xcex7c denotes the rectifying efficiency at the time of charging, and V1 indicates the output voltage from the rectifier circuit and it corresponds to the charging voltage of the large-capacity capacitor 5 in the circuit shown in FIG. 11
Regarding the operating voltage of the processing unit 9 of the portable electronic equipment such as a wristwatch device, ICs or the like are increasingly driven at lower voltage to decrease power consumption; the processing unit may be started, for example, at about 0.9 V to about 1.0 V. Hence, the voltage of the large-capacity capacitor 5 is set to about 1.5 V to about 2 V; when the forward voltage Vf of about 0.5 V to about 0.6 V is considered, the rectifying efficiency xcex7c undesirably takes a value of approximately 0.6. Therefore, it is desirable that the forward voltage Vf is low in order to improve the rectifying efficiency xcex7c.
Further, as the power generating unit which can be built in the portable electronic equipment, there are, for example, a device which employs a rotary weight to catch the movement of a human body or the like and rotates a rotor so as to convert it to AC power, a device which employs a spiral spring to accumulate energy and converts it to AC power, a device which vibrates a piezoelectric element by the movement of a human body or the like in order to obtain AC power, and a device which employs a thermoelectric element or a solar battery to obtain DC power. Among these devices, in the power generating units which provide AC power, the kinetic energy obtained for power generation from the movement of a human body or the like is small, and the electromotive force is also small because the power generator itself is extremely miniaturized to build it in portable electric equipment; and further, the electromotive force greatly varies depending on the movement of a human body or the like and power, therefore, is not always obtained. Thus, the input voltage of the rectifier circuit is low and varies at a voltage in the vicinity of the forward voltage Vf; it reaches only a few times as high as the forward voltage Vf at the maximum. This means that the power feeding efficiency is markedly improved by lowering the forward voltage Vf.
In a power supply apparatus using a power generating unit such as a solar battery which generates DC power, the electromotive force also varies greatly depending on illuminance or the like. Hence, it is possible to effectively use the power, which has been generated, by lowering the forward voltage Vf of a backflow preventing diode even if the illuminance is low and the electromotive force is small. Thus, it is a significant challenge to lower the forward voltage Vf used in the supply circuit in order to effectively use the power supplied from the power generating unit which can be incorporated in portable electronic equipment for which the development is being pursued in recent years.
It is, therefore, an object of the present invention to provide a power supply apparatus with high power feeding efficiency, which power supply apparatus employs a unidirectional unit capable of lowering the forward voltage Vf in place of the diodes to markedly improve the rectifying efficiency xcex7c of the power supply apparatus for portable electronic equipment. And, it is another object of the invention to provide portable electronic equipment which can be used at any place whenever necessary without the need for replacing a battery by providing itself with the power generating unit with high efficiency and a processing unit.
To these ends, in a power supply apparatus which can be built in portable electronic equipment in accordance with the present invention, a unidirectional unit equipped with a diode, a bypass switch connected in parallel to the diode, and a controller which turns ON the bypass switch when a forward voltage appears in the diode is employed for the supply unit that supplies power from a power generating unit to a charging device or a processing unit. In the unidirectional unit, if current flows in the forward direction of the diode and a forward voltage appears, then the bypass switch is turned ON, so that the loss caused by the forward voltage can be prevented. Moreover, if the flowing direction of current is opposite from that of the diode, then a voltage of an opposite polarity from the forward voltage appears; therefore, the bypass switch is not turned ON and backflow can be prevented by the diode.
As a popular unidirectional unit mentioned above, there is a field-effect type transistor; the field-effect type transistor functions as the bypass switch; and a parasitic diode of the bypass switch functions as the diode.
When a power generating unit such as a solar battery employed for a wristwatch device or a power generating system having a rotary weight is used which catches the energy around a user of such a device to generate power, it is less likely that the power generating unit continuously works. It is therefore desirable to make an arrangement so that control may be conducted at a voltage that is higher than the voltage supplied from the large-capacity capacitor which has been charged or the output voltage of the power generating unit which has been stepped up by a booster circuit. By enabling the control at a voltage higher than the output voltage of the power generating unit, it becomes possible to quickly and securely perform switching operation even in the early stage or the ending stage of power generation when the output voltage is low, thus permitting even higher rectifying efficiency.
In such a unidirectional unit, turning the bypass switch ON causes the forward voltage to drop; hence, it is desirable to detect the direction of current in a different method to turn the bypass switch OFF. For instance, the presence of the forward voltage can be detected (sampled) so as to determine the direction of current by the controller first turning the bypass switch ON, then turning the bypass switch OFF after a predetermined time has elapsed. Hence, the presence of the forward voltage is detected periodically, and if the forward voltage is present, then the bypass switch is turned ON again, thus making it possible to decrease the loss of the forward voltage and also to prevent the backflow of current.
It is also possible to detect the direction of current by providing the controller with a comparing means for comparing the voltages at both ends of the diode so as to detect a drop in voltage through the bypass switch while the bypass switch is ON. Further, it is also possible to connect a resistor of an extremely small resistance value in series to the bypass switch in order to cause a voltage drop which can be detected by the comparator. Thus, the adoption of the unidirectional unit which controls the bypass switch depending on whether the forward voltage has been generated in the diode makes it possible to control the unidirectional unit according to the power generating state of the power generating unit without increasing the interfaces with the power generating device. Accordingly, the power generating unit itself no longer requires a coil or the like for detecting an electromotive force, enabling the control for obviating the loss attributable to the forward voltage to be achieved without complicating the configuration of the power generating unit and without increasing the interfaces with the power generating unit.
Further, in order to enable the unidirectional unit to display its performance in a state where no power for controlling the bypass switch is available, it is preferable to adopt an enhancement type field-effect transistor as the bypass switch. The use of the enhancement type makes it possible to fulfill the function of the unidirectional unit to prevent backflow by employing the diode since the bypass switch is turned OFF when no gate voltage is applied.
When AC power is supplied from the power generating unit, the AC power is rectified through the supplying section of the power supply apparatus; therefore, the loss attributable to the forward voltage of the diode can be reduced by using such a unidirectional unit. In particular, the electromotive force of the power generating unit which can be incorporated in portable electronic equipment is small and close to the forward voltage of the diode, so that the rectifying efficiency can be considerably improved, making it possible to provide a power supply apparatus with higher power feeding efficiency.
When performing full-wave rectification in the supply section, four unidirectional units may be used in place of the diode; for first and second input terminals connected to the power generating unit and first and second output terminals connected to the charging unit or the processing unit, first and second unidirectional units may be connected in parallel between the first and second input terminals and the first output terminal, and first and second field-effect transistors may be connected in parallel between the first and second input terminals and the second output terminal. The first conductive field-effect transistors are adopted for the first and second unidirectional units. The first and second field-effect transistors are of the second conductive type; the voltage of the second input terminal is applied to the gate input of the first field-effect transistor, while the voltage of the first input terminal is applied to the gate input of the second field-effect transistor. This causes both the unidirectional units and the first and second field-effect transistors to turn ON/OFF according to the voltage changes at the first and second input terminals, so that the loss due to the forward voltage is eliminated, leading to markedly improved power feeding efficiency.
The timing accuracy of turning the field-effect transistors ON can be improved by connecting driving elements such as inverters to the gate inputs of the second conductive type first and second field-effect transistors. It is also possible to provide a three-input comparator for comparing the voltages at the first and second input terminals with the voltage of the first output terminal as the controller of the first and second unidirectional units so as to enable the respective unidirectional units to determine the forward voltage at the same time, thus making it possible to reduce the total number of the comparators. This permits reduction in the power consumption of the semiconductor device which implements a power supplying circuit. Moreover, since the circuit is simplified, the area of the semiconductor device is reduced with resultant lower cost.
Further, also in the case of the power supply apparatus which has a power generating unit for supplying DC power, the adoption of the unidirectional units mentioned above for preventing backflow permits a reduction in the loss attributable to the forward voltage of diodes. More specifically, the use of the foregoing unidirectional units permits the prevention of the loss due to the forward voltage of the diodes at the time of power generation. In addition, the backflow to the power generator can be also prevented when no power is being generated or the electromotive force becomes lower than the voltage of the charging device.
Thus, the power supply apparatus in accordance with the present invention can be built in portable electronic equipment such as arm-worn type electronic equipment and it has a power generating unit which employs an electromagnetic power generator or a piezoelectric element or the like to output AC power, or a power generating unit such as a solar battery or a thermoelectric device or the like which outputs DC power; it is able to reduce the loss of the power from these power generating units when supplying the power to the charging unit or the processing unit. These power generating units are portable and capable of catching the movement or vibration of the body of a user thereof to generate power or converting discontinuous energy such as sunlight or temperature difference in the natural world into electric energy; it is not, however, capable of continuously providing power, and the electromotive force or the current density is low. Hence, the power supply apparatus in accordance with the present invention prevents the loss attributable to the forward voltage of the diode which nearly corresponds to the electromotive force of the power generating unit when performing rectification, or it prevents backflow in supplying power to the charging unit or the processing unit, thus making it extremely useful as a power supply apparatus for portable electronic equipment. Thus, the use of the power supply apparatus in accordance with the present invention makes it possible to provide full-fledged portable electric equipment such as a arm-worn type equipped with a processing device having a timing function, the electric equipment being able to implement such a processing function as a timing function at any place whenever necessary.